1. Field of the Invention
The present invention relates generally to the screen process printing and coating industry and more specifically to an improved drying apparatus which provides for minimum energy and floor space requirements in high volume screen process printing and coating operations by uniquely combining hot air and radio frequency drying in a sequential conveyer belt arrangement.
2. Prior Art
The recent increase in ecological and work safety concerns has motivated the increased use of water-based inks and coatings in the screen process printing industry. Although the drying or curing of such water-based inks and coatings may be carried out at ambient temperatures at relatively high production volume, such as 2,000 to 3,000 impressions per hour, the storage space requirements to permit an adequate opportunity for 100% curing are prohibitive because each such sheet must be fully dried before it can be stacked. Stacking before a 100% cure has been achieved can lead to offset interblocking wherein the incompletely cured ink or coating on sheet attaches to the next sheet in such stacking, particularly where such stacking is done in significant numbers, such as in high volume production stacking. One prior art technique for accelerating the curing time for water-based screen process coatings and inks has been the use of high velocity jet dryers which typically propel hot air, such as air at 200 to 250 degrees Fahrenheit, at the coated or printed sheets as they leave the coating apparatus. Unfortunately, in high volume screen process coating or printing production lines, employing 2,000 to 3,000 impressions per hour, it is typically necessary to provide up to 40 feet of such high velocity jet dryer apparatus and an additional 20 feet or more of cooling apparatus in order to attain the degree of curing necessary to avoid the aforementioned offset interblocking during stacking. Accordingly, a great deal of floor space is required to provide the necessary amount of high velocity jet drying action. Furthermore, this floor space problem is further exacerbated by the need to provide large cooling systems which typically must occupy 15-20 feet of floor space in order to offset the affect of the high velocity jet dryer wherein the extremely hot air has heated not only the coating, but also the underlying sheet material. Still another disadvantage of the exclusive use of large high velocity jet dryers for drying water-based coatings is the extremely high energy requirements of such jet dryers. For example, in a typical prior art application where a 40 foot long high velocity jet dryer producing high velocity air at approximately 250 degrees Fahrenheit is utilized, energy requirements would be on the order of 100-120 kilowatts. Furthermore, the total floor space requirement for such inefficient hot air drying would be on the order of 100 linear feet.
It will be seen hereinafter that the present invention utilizes a novel combination of a significantly reduced amount of hot air drying and a radio frequency (RF) dryer, to significantly reduce both the power requirement and the floor space requirement for drying water-based screen process printing or coating applications. The use of RF drying is not novel for removing water from materials in manufacturing processes. RF drying for example, has been used in the food processing industry to extract moisture from food products without damaging such products. It has also been used in the stationary industry, for example, to adhere glue on envelopes, stamps and the like. However, it is believed to be an entirely novel and unique application of RF drying to the screen process printing industry, particularly in combination with a significantly reduced amount of hot air drying which precedes the RF drying for purposes of accelerating the curing prior to the RF drying stage. Consequently, until the present invention described herein, there has been an ongoing need for a more efficient method for drying water-based screen processing materials and particularly for curing the coatings and inks thereon, in order to provide a system which can generate a 100% cured high volume production screen printing process in volumes of at least 2,000 impressions per hour while significantly reducing the amount of floor space and the energy requirement for such production.